1. Field of the Invention
This invention is concerned with a device for controllably heating objects during a phase of a manufacturing operation.
2. Prior Art
In the electronics industry, it is often necessary to heat components during various phases of manufacturing operations. For example, infrared ovens are used to cure photoresists on silicon wafers and ribbons, ceramic substrates, printed circuit boards, and glass substrates, as well as to cure polymides/polyurethane coatings on such substrates. Such ovens are also used to drive out organic solvents in anti-reflection coatings, spray-on/spinners, dopant coatings, thick film pastes, silkscreen resists and emulsion coatings. Infrared ovens providing even higher temperatures are used for the sintering of thick film metal pastes such as aluminum, silver and palladium, the high temperature baking of silicon, ceramics and other substrates, the dopant drive-in for silicon substrates, and the drying of anti-reflection coatings, to mention only a few of the many uses of heat in various phases of manufacturing processes in the electronics industry. Many of the uses of heat in the processing of electronics components require that the processing temperature be profiled, i.e., the electronics components be put through a heating cycle in which the temperature is varied at different points within the cycle.
The use of ovens which radiate infrared energy to objects in order to heat the objects is accompanied by a number of shortcomings. One of these is the ability to accurately measure the temperatures actually reached by the objects being heated. For example, during the curing of polymide coatings on silicon in infrared ovens, it has been found that the actual temperatures can be 40 to 50 percent more than indicated by thermocouples as conventionally used. This can result in blistering and non-uniformity in the cured coatings, and is highly undesirable.
Another shortcoming is the large amount of energy required by infrared ovens and the concommitant heat loss to the environment during heating cycles which often require significant amounts of time.
A further shortcoming is the inadaptability of conventional heating methods to mass production techniques. While many, if not most, of the other steps in many manufacturing operations may be automated in a manner which employs assembly line techniques, the heating of objects as part of a manufacturing process remains essentially a batch process.
More rapid and efficient heating of objects has long been known to be a characteristic of the use of microwave energy for heating, the microwave energy being in the wavelength range of 300 MHz to 300 GHz. However, the ability of an object to be heated by direct coupling of microwaves into it depends on the composition of the object. Certain materials, such as silicon, are microwave absorptive and can be directly heated by microwave energy. Other materials are not microwave absorptive; such materials will not be appreciably heated as direct coupling of microwave energy into such materials does not occur or does not occur to a great enough extent. Quartz is an example of such a material.
As microwave energy represents a potentially clean, quick and efficient method of heating objects, it is an objective of the present invention to use microwave energy to heat objects during various phases of manufacturing processes. It is also an objective of the present invention to provide a heating system using microwave energy that can successfully heat, according to a predetermined temperature profile, both objects which are microwave absorptive, and those which are not. It is yet a further objective of the present invention to provide such a system which is easily adaptable to automated manufacturing techniques rather than restricted to essentially a batch mode of operation.